FIFA has threatened to ban any player or club involved in breakaway competitions, amid renewed talk of a European Super League involving some of the world's biggest teams.
A breakaway continental tournament has been mooted and discussed for years, with Barcelona, Real Madrid and many leading Premier League clubs believed to favour a ring-fenced European league which would include some of the richest and most famous teams.
However, this has been vocally rejected by others, including Bundesliga giants Bayern Munich, while UEFA president Aleksander Ceferin and La Liga chief Javier Tebas are among the most ardent critics of the breakaway plans.
Now FIFA has made its stance clear, saying that players will be banned from playing in any of their competitions - including the World Cup and European Championship - should they take part in any unauthorised events.
The letter has been signed by the six heads of FIFA's continental confederations and reads: "In light of recent media speculation about the creation of a closed European 'Super League' by some European clubs, FIFA and the six confederations once again would like to reiterate and strongly emphasise that such a competition would not be recognised by either FIFA or the respective confederation.
"Any club or player involved in such a competition would as a consequence not be allowed to participate in any competition organised by FIFA or their respective confederation.
"As per the FIFA and confederations statutes, all competitions should be organised or recognised by the relevant body at their respective level, by FIFA at the global level and by the confederations at the continental level.
"In this respect, the confederations recognise the FIFA Club World Cup, in its current and new format, as the only worldwide club competition while FIFA recognises the club competitions organised by the confederations as the only club continental competitions.
"The universal principles of sporting merit, solidarity, promotion and relegation, and subsidiarity are the foundation of the football pyramid that ensures football’s global success and are, as such, enshrined in the FIFA and confederation statutes.
"Football has a long and successful history thanks to these principles. Participation in global and continental competitions should always be won on the pitch."
The statement is then signed off with the names of the presidents of the six supporting confederations, as well as FIFA president Gianni Infantino.